Amplifying fibers, particularly optical fibers doped with rare earth elements may be used for numerous optical applications.
For example, erbium doping is used in long-distance optical telecommunication systems for amplifying transmitted optical signals. Such optical fibers are used in erbium-doped fiber amplifiers (i.e., EDFAs). Erbium-doped fiber amplifiers can have a central core composed of a silica matrix that includes doping elements, such as erbium at concentrations of about 250 ppm to 1000 ppm (0.025 to 0.1 weight percent), optionally combined with complementary doping elements that make it possible to improve amplification (e.g., alumina for broadening the gain bandwidth for wavelength dense multiplexing (WDM) applications).
Ytterbium doping is often used in optical fibers for powerful laser applications. The ytterbium concentrations in such optical fibers are high (e.g., several weight percent). Ytterbium can also be used in the erbium-doped fiber amplifiers to improve the effectiveness of absorption of the pump signal by the erbium.
Optical amplification in a rare-earth-doped optical fiber is achieved by injecting into the optical fiber a pump signal, which excites the rare earth ions (e.g., Er3+ in EDFA). When a light signal passes through this portion of optical fiber, it de-energizes the ions by stimulated emission by producing a photon identical in all respects to the incident photon. The light signal is thus doubled. A portion of such an optical fiber in combination with a resonant cavity constituted by a system of mirrors, or Bragg gratings, forms an optical-fiber laser. The wavelength and power of the laser depend on the rare earth element used and its concentration.
For certain applications, it is desirable to obtain high powers at the amplifying fiber output. It is also increasingly desirable to reduce the size of optical systems, and so research is being carried out with respect to compact optical fibers. With respect to reduced-size optical fibers, it may be necessary to increase the concentration of rare earth dopants in the optical-fiber core to increase the amplification gain.
If the concentration of rare earth dopants in the optical-fiber core is significant, the formation of packets of ions in the silica matrix of the core is observed. These packets create doping inhomogeneities, which impair the effectiveness of the emission of each rare earth ion. This results in a limitation of the amplification gain due to the existence of certain concurrent de-energizing paths between adjacent ions (e.g., transfers of energy by cross reactions). Such ion clusters also accentuate the photon degradations that can occur in the core of the high-power optical fiber during the propagation of light signals in the optical fiber. In fact, the crystalline defects present in the silica matrix of the core can absorb the energy from the photons emitted by rare earth ions that are de-energized. This can create darkening points in the core, which give rise to additional losses. Defects in the silica core matrix in the vicinity of rare earth clusters promote the formation of darkening points, because such silica defects can readily absorb the energy (e.g., ultraviolet light) emitted by such rare earth clusters.
The publication of “Liekki White Paper Photodarkening: Understanding and Mitigating,” Koponen et al., (March 2005), which is incorporated by reference, identifies the problem of the photodarkening of rare-earth-doped optical fibers. This publication correlates the problem of photodarkening with the formation of rare-earth-ion clusters in the core and proposes to limit the inhomogeneities by using a method of production by direct deposition of nanoparticles (NPs) by a so-called direct nanoparticle deposition (DND) process.
The production method described in this publication is an alternative to modified chemical vapor deposition (MCVD), which is often used for doping the optical-fiber core. This publication proposes a production method based on an outside vapor deposition (OVD) technique in which the formation of the silica and the doping are carried out simultaneously. Nanoparticles composed of a rare-earth-doped silica powder, optionally co-doped with other elements, are formed when the reagents are simultaneously injected into the flame of a torch, and then directly projected to form a rod of doped silica constituting the optical-fiber core. Such a production method, however, does not make it possible to preserve the structure of nanoparticles in the optical-fiber core. These nanoparticles are simply particles of doped silica, such as those obtained in a vapor deposition method, before being fused at high temperature in order to form the layers of glass that provide the primary preform. This publication observes that this outside vapor deposition technique makes it possible to obtain a better homogeneity of the rare earth dopants in the optical-fiber core compared with an MCVD impregnation technique.
Nonetheless, the rare earth ions can still be found close to crystal defects in the silica matrix of the optical-fiber core. Moreover, the appearance of darkening points in the optical-fiber core is not completely avoided when the optical fiber is used to transmit high-power optical signals and has a high concentration of rare earth dopants.
Rare earth dopants can be introduced into the optical-fiber core by incorporating nanoparticles doped with rare earth elements via MCVD. For example, (i) European Patent No. 1,347,545 and its counterpart U.S. Pat. No. 7,031,590 and (ii) International Publication No. 2007/020362 and its counterpart U.S. Patent Publication No. 2009/0116798, each of which is incorporated by reference, describe optical fibers having nanoparticles in the optical-fiber core. The disclosed nanoparticles include a rare earth doping element and at least one element that improves the amplification of the signal (e.g., aluminum, lanthanum, antimony, and bismuth). The characteristics of the nanoparticles (e.g., design, composition, size, concentration) and of the doping described in these patent-related documents do not ensure the reduction or elimination of photodarkening in the optical-fiber core for high concentrations of rare earth ions.
Therefore, there is a need for an amplifying optical fiber that is highly doped with rare earth elements and that allows the use of high optical powers without damaging the optical fiber.